In a power transmission mechanism configured to transmit the rotary motion of a crankshaft via an endless traction means, e.g. a chain, to one or more camshafts or other units of an internal combustion engine of a motor vehicle, the tension of the endless traction means is held constant by providing a tensioning device which exerts a tensioning force on the endless traction means. This tensioning force, which is transmitted to the endless traction means by a plunger of the tensioning device, can be provided by a cylindrical compression spring, for example. If there is a sag in the endless traction means, the plunger preloaded by the compression spring moves forwards in order to exert a pressure on the endless traction means, thus ensuring that the endless traction means once again rests with the necessary tension against the drive-input and drive-output pulleys of the crankshaft and of the camshafts or units and cannot slip.
The cylinder chamber of the tensioning device, in which the plunger is guided, can furthermore be supplied with a pressure medium, with the result that the tension of the endless traction means can be controlled by varying the tensioning force exerted by the plunger, and the retraction motion of the plunger is damped with the aid of a hydraulic oil fed in behind the plunger. The flow of hydraulic oil from an oil space situated in the housing to the plunger is generally regulated by a nonreturn valve.
In order to avoid slipping or skipping of teeth or driven gearwheels under shock loading, the tensioning device can have a latching system. The latching system is intended to prevent the plunger from being pushed too deep into the housing under shock loading, e.g. when the engine is started, resulting in a slackening of the tension on the endless traction means. One particular latching system is described in publication EP 1 498 580 B1, for example. This is a detent pawl latching system, in which a spirally toothed latching sleeve is provided within the plunger, into which latching sleeve an engagement element connected to the plunger engages. The toothing of the latching sleeve defines possible latching stages, as far as which the plunger can move back into the housing.
A cylindrical latching system of this kind requires a relatively large length of installation space, for which reason it is of only limited suitability for applications in which only a small amount of installation space is available. If the latching system is made smaller, it generally also has a reduced retraction stroke.